A conventional squirrel-cage motor rotor is composed of a cylindrical rotor core formed by stacking electromagnetic steel sheets, a plurality of conductor bars provided in respective slots of the rotor core, and short-circuit rings (end rings) connected to the ends on both sides of the conductor bars, and the rotor is attached to a rotor shaft of a motor by shrink fit.
In recent years, a squirrel-cage motor used for a spindle motor of a machine tool, or the like, is required to have high performance, and is required to be capable of high-speed rotation at several tens of thousands of revolutions/minute.
In order to improve the efficiency of such a squirrel-cage motor that rotates at a high speed, aluminum comes to be used for the conductor bars and the short-circuit rings which are conductors of the rotor (hereinafter, referred to as rotor conductors).
The squirrel-cage motor rotor that rotates at a high speed is, due to the structure thereof, subjected to a great centrifugal force when rotating at a high speed. In addition, due to heat generation during operation, the rotor conductor thermally expands. That is, due to the action of the centrifugal force and the thermal expansion, the short-circuit ring is deformed, and stress occurs on the short-circuit ring.
When the squirrel-cage motor is repeatedly driven and stopped, application and cancellation of stress on the short-circuit ring are repeated. That is, the short-circuit ring is repeatedly subjected to stress, to undergo metal fatigue.
Particularly, in the case of using, as the rotor conductors, aluminum which is low in strengths such as proof stress and tensile strength and is comparatively great in the coefficient of thermal expansion, deformation of the short-circuit ring increases, and the caused stress also increases. In addition, there is a possibility of reduction in the fatigue strength of the short-circuit ring.
A squirrel-cage motor rotor is proposed which can prevent deformation of the short-circuit ring due to a centrifugal force and heat though aluminum is used for rotor conductors. This rotor includes aluminum-cast-type rotor bars (conductor bars) formed by being cast into outer circumferential slots of a rotor iron core (rotor core); and aluminum-cast-type short-circuit rings provided at both ends of the rotor bars. A holding ring made of steel or the like is attached to the outer circumference of each short-circuit ring by shrink fit, and a plate lock (reinforcement cover) for holding and reinforcing the outer ends of the short-circuit ring and the holding ring is provided in a rotor shaft via a boss (for example, see Patent Document 1).
Another squirrel-cage motor rotor is proposed in which, although aluminum is used for rotor conductors, the amplitude of the repetitive stress caused by repetition of driving and stop is reduced, thus preventing metal fatigue. This rotor includes a cylindrical iron core portion (rotor core); conductor bars arranged at constant intervals along the circumferential direction inside the iron core portion; short-circuit rings connected to the conductor bars and provided at both end surfaces of the iron core portion; and reinforcement rings (reinforcement rings) the outer diameter of which is greater than the inner diameter of the short-circuit rings. The reinforcement ring is shrink-fitted to the inner circumference of each short-circuit ring, and a shaft (rotor shaft) is attached to the inner circumferences of the iron core portion and the reinforcement ring (for example, see Patent Document 2).